Never a Dull Moment with Woolly Nightshade!

A number of additional species have been found attacking woolly nightshade (Solanum mauritianum) in New Zealand since we undertook comprehensive surveys of its natural enemies in 2000/01.

These include two fungi (Phoma glomerata and Fusarium sambicinum), a bacterium (Erwinia persicinus), and a psyllid (Acizzia solanicola). The latter was found by Quentin Paynter earlier this year in Auckland on a woolly nightshade plant in his home garden on which he was cultivating lace bugs (Gargaphia decoris). “Since I know the fauna of woolly nightshade well I realised this was an unusual find, and I reported it to the Ministry for Primary Industries (MPI),” explained Quent. MPI confirmed the psyllid was a new incursion for New Zealand, and potentially a threat to eggplant (Solanum melongena), but it was already too widely established to consider eradication or containment efforts. However, MPI are interested to know about any further sightings of this new incursion. The psyllid is thought to be native to Australia, and it is not known how it got here.

Interestingly, the potato/tomato psyllid (Bactericera cockerelli) has not yet been found on woolly nightshade in New Zealand. This serious new pest was first found in New Zealand in 2006 and is still spreading. This psyllid’s host-range includes a range of Solanum species and other genera including Ipomoea and Convolvulus, so they may well be found on woolly nightshade in due course.

Unfortunately none of these pathogens or insects lend themselves to being used for biocontrol since they also attack desirable plant species. Fortunately some woolly nightshade specialists do exist. Releases of the first biocontrol agent for woolly nightshade in New Zealand got underway in November 2010. Since that time 30 populations of the lace bug (Gargaphia decoris) have been released in Northland, Auckland, Bay of Plenty, Waikato, Taranaki, Manawatu-Wanganui, Gisborne and Tasman. Detailed follow-up of their establishment success will be undertaken once the lace bugs have had a little longer to settle in, but we have received some encouraging reports already that suggest establishment is occurring.

The second agent to be considered for New Zealand is a flowerbud-feeding weevil (Anthonomus santacruzi). Weevil host testing has been undertaken for us by Terry Olckers and some of his students at the University of KwaZulu-Natal, in South Africa. Terry has been responsible for the biocontrol programme for woolly nightshade in South Africa for many years and a great help to our New Zealand project. Flowerbud-feeding weevils were first released in South Africa in 2008, nearly a decade after lace bug releases fi rst began there. Both agents have established and are dispersing. One damaging outbreak of the lace bug has been observed but the site was unfortunately destroyed by fi re soon after. “It is unclear at present if these two agents will be able to do the job for South Africa or if others will be needed,” confirmed Terry.

All potential woolly nightshade biocontrol agents have shown a tendency, during host testing in cages, to attack plants that they avoid under more realistic, natural conditions (this is known as false-positive results). We were therefore not surprised when Terry told us that the flowerbud-feeding weevil had attacked one of our native poroporo species (Solanum aviculare) in cage tests. We asked Terry if he could do some field tests for us and he willingly obliged. Unfortunately the result was the same. While poroporo was a vastly less preferred host than woolly nightshade the weevil did complete its life cycle on this species in the field. The weevil is therefore not an acceptable biocontrol agent for New Zealand, and we will not be proceeding any further with it. While it is disappointing for an agent to fail at the final hurdle it is comforting to know that host-testing methods can identify agents that pose unacceptable risks. The next task is to decide what potential agent to study next. The two top contenders are likely to be another fl owerbud-feeding weevil (Anthonomus morticinus) and a stem-boring weevil (Conotrachelus squalidus), both of which occur in southern Brazil.